Stylus profilers are used for obtaining surface profiles of samples. The stylus of the profiler follows the surface under a small contact force, and the resulting motions of the stylus are measured with a sensor assembly. The sensor assembly includes a stylus, a mechanical linkage (usually a stylus arm) connecting the stylus to a flexure pivot, and a transducer. When the stylus is scanned across the surface of the sample, the force exerted by the sample surface on the stylus causes a rotation of the stylus arm about the flexure pivot. The vertical displacement of the stylus is converted by the transducer into an electrical signal which indicates the profile of the sample surface.
Advanced profilers also include a force control mechanism, such as an electromagnetic actuator, for maintaining a constant contact force between the stylus and the sample surface as the stylus is scanned across the surface. To maintain a constant contact force between the stylus and the sample surface, the spring action of the flexure pivot is calibrated and the force control magnetic actuator is controlled to counteract the change in the force applied by the flexure spring on the stylus caused by rotation of the stylus arm. Thus, a constant force is exerted by the stylus against the sample surface, as the stylus is scanned across the surface. As an example of a profiler which has been used in the semiconductor and disk drive industries, please see U.S. Pat. No. 5,705,741 and U.S. Pat. No. 5,309,755; both patents are incorporated herein in their entirety by reference.
As the semiconductor industry progresses to smaller dimensions with each new generation of products, there is an increasing need for scanning instruments that can measure sub-micrometer scale surface features. While the depths or vertical dimensions (dimensions normal to the plane of the wafer surface) of the features such as trenches, or via holes, in semiconductor wafers, commonly exceed one micrometer, the lateral dimensions (dimensions in the plane of the wafer surface) have been continually reduced. At the current state of the art, the lateral dimensions of features such as trenches are less than 0.5 micrometer. With the continual reduction of the lateral dimensions of features such as trenches and via holes in the surface of semiconductor wafers, the ratio of depth to the lateral dimension of such features, also known as the aspect-ratio, is continually increased.
In order to measure such high aspect-ratio features, a very sharp, thin but long (high aspect-ratio) stylus must be used. However, a sharp, thin but long stylus is fragile and may easily break, especially when subjected to lateral forces (forces in directions in, or parallel to, the plane of the sample surface). Thus, when a high aspect-ratio stylus contacts a steep feature, such as the side wall of a trench or via hole, the contact force has a relatively large lateral component and a relatively small vertical component. Stylus profilers, such as the profilers described in the two patents referenced above, are designed such that motion of the stylus is constrained to one degree of freedom, namely, rotation about the flexure pivot. This degree of freedom is substantially normal to the sample surface. The stylus arm is relatively stiff in all other degrees of freedom. Consequently, the lateral forces generated when the high aspect-ratio stylus encounters a steep wall can easily break the stylus and damage the sample being measured.
The stylus arm in a profiler has a single degree of freedom, which comprises rotations about a pivot. The stylus or sensing tip travels along a path normal to a radial line passing through the center of rotation at the pivot and the tip. Since the sensing or stylus tip must be located “below” or at a lower elevation than the pivot to ensure that the tip and not the body of the sensor assembly contacts the sample, the motion of the stylus or sensing tip is not truly normal to the plane of the sample surface, but is in the shape of an arc. While the main direction of travel of the tip is downwards, it nevertheless also travels in the lateral direction in the plane of the sample surface. This lateral motion is also known as parasitic motion of the sensing tip. The parasitic motion of the sensing tip may hamper or even preclude the sensor assembly from measuring relatively deep and narrow features.
It is therefore desirable to provide an improved surface measurement system which overcomes the above drawbacks.